Parametric devices are flexible and convenient sources of widely-tunable coherent radiation, encompassing all time-scales from the femtosecond pulse to the continuous-wave. In these, a coherent beam of electromagnetic radiation is used to stimulate a non-linear process in a non-linear optical crystal, resulting in the division of the power/energy in the coherent pump wave into two generated waves, typically referred to as the signal and idler waves. The signal is usually defined as that wave providing the useful output, and hence throughout this document is identified as the wave having the longer wavelength of the two generated waves.
Parametric devices can operate in a variety of configurations including amplifiers, oscillators and generators. In a parametric amplifier an intense coherent pump wave is made to interact with the nonlinear optical crystal to produce amplification at the signal and idler optical wavelengths. A parametric oscillator uses a parametric amplifier inside an optical cavity resonant at one or both of the signal and idler waves. Here, the signal and idler waves are either self-starting from noise/parametric fluorescence or the cavity is injection seeded by a suitable source operating at the signal and/or idler wavelength. A parametric generator generates optical waves by the interaction of an intense coherent pump wave with a nonlinear optical crystal to parametrically produce two other optical waves. No cavity is provided for the down-converted waves since parametric gain is sufficiently high as to allow adequate transfer of energy/power to these waves with only non resonant single (or multiple) passing of the pump and or idler and or signal waves through the nonlinear medium. Again, in this case the signal and/or idler waves are either self-starting from noise/parametric fluorescence or the generator is injection seeded by a suitable source operating at the signal and/or idler wavelength.
There is considerable interest in extending the spectral coverage of parametric devices. This is because they are often used as sources of coherent radiation in spectral ranges either not covered by any other sources, or where a single parametric-wave source is capable of replacing a number of sources that would otherwise be needed in order to provide the spectral coverage required. However, a serious limitation of known parametric devices is the detrimental effect of absorption of one or more of the three waves involved in the nonlinear interaction within the nonlinear medium itself. As a result the spectral coverage attainable through a particular parametric generation scheme is often limited only by the presence of absorption and not by the nonlinear or phase-matching characteristics of the nonlinear medium being employed. Elimination of the restriction imposed by absorption would result in improved spectral coverage.
One solution for overcoming problems due to absorption has been identified. This involves using non-collinear phase-matching in such a way as to cause the wave subject to absorption, usually the signal wave, to rapidly walk-out of the nonlinear medium in a direction that is substantially lateral to the propagation direction of the pump wave. Examples of this technique are described in the articles “Efficient, tunable optical emission from LiNbO3 without a resonator”, by Yarborough et al, Applied Physics Letters 15(3), pages 102-104 (1969); “Coherent tunable THz-wave generation from LiNbO3 with monilithic grating coupler”, by Kawase et al, Applied Physics Letters 68(18), pages 2483-2485 (1996), and “Terahertz wave parametric source”, by Kawase et al, Journal of Physics D: Applied Physics 35(3), pages R1-14 (2002).
FIG. 1 is an illustration of the known non-collinear phase-matching process. More specifically, FIG. 1(a) illustrates the geometry of the interacting pump 1, idler 2 and signal 3 waves in the nonlinear medium 4. FIG. 1(b) illustrates the phase-matching process through a so-called k-vector diagram, where kp, ki and ks are the wavevectors of the pump, idler and signal waves respectively, angle θ is the angle subtended by the pump 1 and idler 2 waves and angle ρ is the angle subtended by the pump 1 and signal 3 waves.
As can be seen from FIG. 1(b), in the known non-collinear phase matching process the pump wave 1 and idler wave 2 are not themselves collinear within the nonlinear medium 4. However, to maintain the necessary nonlinear interaction between them throughout the length of the nonlinear medium 4, they must be of sufficient radial (transverse) extent to maintain an overlap between them throughout the length of the medium 4. This means that it is not possible to employ small (i.e. tightly focussed) beam sizes for these waves. Having small beam sizes is desirable because it increases the intensities of the waves, so as to reduce the power or energy necessary for attaining a level of parametric gain required for the operation of the device.
WO 2006/010916 describes a parametric device that uses a nonlinear crystal that has slant-stripe-type of periodic-poling to generate a signal and an idler wave in response to being stimulated with a pump wave, wherein the non-linear medium is such that the pump and idler waves are collinear and the signal wave is non-collinear. By arranging for the signal wave to be non-collinear with the other waves, this means that the signal wave walks-off from the other waves and exits the nonlinear medium within a short distance and hence with reduced absorption. Because the pump and idler beams are collinear, tight focussing of these beams is now possible. Hence, the parametric gain available for a given pump power/energy is not restricted, as described previously, by the requirement to maintain relatively large beam sizes for the purpose of ensuring beam overlap throughout the length of the nonlinear medium. An advantage of tight focusing of the pump and idler beams is that these beams may be propagated closer to the edge of the nonlinear medium so further reducing the path over which the signal beam must propagate before exiting the medium. This further reduces the absorption losses to which this beam is subjected. Having the pump and idler waves collinear means that common elements can be used such as, but not restricted to, mirrors for the guidance of these waves. This can simplify otherwise complicated arrangements.
FIG. 2 illustrates the non-linear process that occurs in a slant-stripe-type periodically-poled crystal when it is pumped with a single pump wave. In this case, the pump wave is collinear with the idler wave, but the signal wave walks-off in a direction substantially lateral to the pump wave. The operation of this device and in particular the role played by slant-stripe-type periodic-poling may be described in terms of phase-matching diagrams in which the periodic poling is represented by a grating vector kΛ of length determined by the period of the poling and direction determined by the slant angle of the poling. Incorporation into a vector phase-matching diagram of such a grating vector kΛ allows the geometry associated with the wave-vectors of the pump, signal and idler waves to be elucidated.